The present invention relates to a saturatable absorbant with very short switching times. It is used in optics, particularly in the production of lasers emitting very short pulse trains and in the production of all optical logic gates.
A saturatable absorbant is a material, whose absorption coefficient significantly decreases when a large amount of light is applied to it. This type of material has numerous applications, reference being made in a non-limitative manner to the production of lasers operating under mode locking conditions and emitting pulse trains of an extremely short width below one picosecond and the production of all optical logic gates, in which a first high power light beam controls the passage or stopping of a second light beam.
Saturatable absorbants have long been constituted by amorphous materials, such as dissolved colouring agents. However, for some years now, interest has been attached to a new class of materials called multiple quantum wells or MQW and to variants of these structures called superlattices.
A quantum well is obtained by inserting between two thin films of a first conductive material a thin film of a second material having a smaller forbidden band or gap than that of the first. In this way a potential well is produced for the charge carriers in the central semicoductor having the smallest gap, said well being surrounded by two potential barriers corresponding to the two extreme films. A multiple quantum well is obtained by superimposing such structures, without there being any coupling between the wells (which is obtained by giving the barriers an adequate thickness).
A superlattice is obtained when the potential barriers are sufficiently thick for there to be a coupling between the different wells. Thus, a superlattice is formed from a stack of films of two different semiconductor materials having gaps of different heights. Potential wells are produced in the films corresponding to the semiconductor with the smallest gap and a potential barrier appears in each film corresponding to the semiconductor with the largest gap.
FIG. 1 diagrammatically illustrates said structure and its properties. It is possible to see in part (a), a stack of films of two semiconductor materials SC.sub.1 and SC.sub.2. The energy level diagram is shown in part (b), where G.sub.1 and G.sub.2 represent the gaps separating the valence band at the bottom from the conduction band at the top. It is assumed in FIG. 1 that semiconductor SC.sub.1 has the smallest gap and consequently the potential wells are formed in this material. These wells have a width Lp corresponding to the thickness of the corresponding films. In the films of semiconductor SC.sub.2 having a thickness Lb are formed the potential barriers. These wells are occupied by electrons in the conduction band and by holes in the valence band.
Numerous publications deal with such superlattices. Reference is e.g. made to two general articles entitled "solid state superlattices" published G. H. DOHLER in Scientific American, November 1983, Vol 249, No 5, pp 144 to 151 and "Les super-reseaux artificiels" published by J. F. PALMER in L'Echo des Recherches, No 105, July 1981, pp 41 to 48. In connection with quantum wells, reference is also made to the article by R. M. KOLBAS et al entitled "Man-made quantum wells: a new perspective on the finite square-well problem" published in the American Journal of physics, 52 (5), May 1984, pp 431-437.
Such materials have a double structural perodicity, one due to the crystalline structure of the semiconductors used and the other due to the regular stacking of the films. Thus, both in the valence band and in the conduction band there are discrete energy levels (or microbands) which are offered to the holes and electrons. FIG. 1 diagrammatically shows a level Ee in the conduction band, which can be occupied by an electron (e) and a level Eh located in the valence band, which can be occupied by a hole (h).
The position of these energy levels is obviously dependent on the materials used and on the thickness of the films.
There is great interest in such structures. This has increased since it was found that they had saturatable absorption lines. Absorption taken place in the films corresponding to the semiconductor with the smallest gap, where the wells are located. Since this discovery these devices have been used in mode locking lasers. Thus, it has been possible to produce a semiconductor laser emitting a pulse train of width equal to 1.3 ps with a recurrence frequency equal to 1 GHz. The passive locking process is obtained as a result of a external resonant cavity and a superlattice of the multiple quantum well type bonded to one of the mirrors of the cavity. This is described in the article by Y. SILBERBERG et al entitled "Passive mode locking of a semiconductor diode laser" published in Optics Letters, November 1984, Vol 9, No 11, pp 507-509. These devices have also been used in optical logics. It has been possible to produce a logic NOR gate having a switching time below 1 picosecond. Such an application is described in the article by A. MIGUS et al entitled "One-picosecond optical NOR gate at room temperature with a GaAs-AlGaAs multiple-quantum-well non linear Fabry-Perot etalon" published in Applied Physics Letters, 46 (1), January 1985, pp 70-72.
The establishment of the phenomena involved in these applications is extremely short and is approximately or less than 1 picosecond, which is the switching time which can be called "on" and which represents the absorption saturation. However, the return to equilibrium, which characterises a switching time which can be called "off" is much longer and is approximately 1 nanosecond.
It is widely accepted that it is the latter time which essentially limits the performances of such devices. Various solutions have been proposed for obviating this deficiency and particularly the irradiation of the absorbant by electrons, which reduces the radiative life in the structure and therefore the return to equilibrium time. However, this is difficult and complex to carry out.